Free-cutting Ni-base heat-resistant alloy

ABSTRACT

A free-cutting Ni-base heat-resistant alloy excellent in the high-temperature strength and corrosion resistance was proposed. The alloy contains Ni as a major component, 0.01 to 0.3 wt % of C and 14 to 35 wt % of Cr, and further contains at least one element selected from Ti, Zr and Hf in a total amount of 0.1 to 6 wt %, and S in an amount of 0.015 to 0.5 wt %. The alloy has dispersed in the matrix thereof a machinability improving compound phase, where such phase contains any one of Ti, Zr and Hf as a major constituent of the metal elements, essentially contains C and either S or Se as a binding component for such metal elements. The alloy also satisfies the relations of W Ti +0.53W Zr +0.27W Hf &gt;2W C +0.75W S  and W C &gt;0.37W S , where W Ti  represents Ti content (wt %), W Zr  represents Zr content (wt %), W Hf  represents Hf content (wt %), W C  represents C content (wt %) and W S  represents S content (wt %). This successfully suppresses the amount of free S residing in the alloy, which results in an improved machinability while preventing the hot workability from being degraded.

RELATED APPLICATION

[0001] This application claims the priority of Japanese PatentApplication NO. 2001-167940 filed on Jun. 4, 2001 which is incorporatedherein by reference.

FIELD OF THE INVENTION

[0002] The present invention relates to a free-cutting Ni-baseheat-resistant alloy having an excellent machinability.

BACKGROUND OF THE INVENTION

[0003] An excellent high temperature strength is demanded for exhaustvalves and bolts for engines since they are used under high temperatureenvironment. There is an additional demand of corrosion resistanceagainst exhaust gas for exhaust pipes and valves in chemical plants aswell as the demand of high temperature strength. It has thus been ageneral practice to use, as a structural material for composing suchparts, nickel (Ni)-base heat-resistant alloys excellent in strength andcorrosion resistance in high temperature ranges.

[0004] A problem of poor machinability has, however, resided in theconventional Ni-base heat-resistant alloy, although being excellent inthe strength and corrosion resistance. Structural steel or stainlesssteel will successfully be improved in the machinability by being addedwith so-called machinability improving elements such as Pb, Bi, S, Se orTe, but the Ni-base heat-resistant alloy will considerably be ruined inthe hot workability by containing such machinability improving elements.So that almost no approach has been made for Ni-base heat-resistantalloy to intentionally improve the machinability, which has inevitablypushed up machining costs of such material in the product making.

[0005] It is therefore an object of the present invention to provide afree-cutting Ni-base heat-resistant alloys excellent in strength andcorrosion resistance in high temperature ranges and in machinability.

SUMMARY OF THE INVENTION

[0006] To solve the foregoing problems, a free-cutting Ni-baseheat-resistant alloy of the present invention contains Ni as a majorcomponent;

[0007] contains C in an amount of 0.01 to 0.3 wt % and Cr in an amountof 14 to 35 wt %;

[0008] contains at least one element selected from Ti, Zr and Hf in atotal amount of 0.1 to 6 wt %, and S in an amount of 0.015 to 0.5 wt %;

[0009] has dispersed in the matrix thereof a machinability improvingcompound phase, where such phase contains any one of Ti, Zr and Hf as amajor constituent of the metal elements, essentially contains C andeither S or Se as a binding component for such metal elements; and

[0010] satisfies the relations of:

W_(Ti)+0.53W_(Zr)+0.27W_(Hf)>2W_(C)+0.75W_(S); and

W_(C)>0.37W_(S)

[0011] where W_(Ti) represents Ti content (wt %), W_(Zr) represents Zrcontent (wt %), W_(Hf) represents Hf content (wt %), W_(C) represents Ccontent (wt %) and W_(S) represents S content (wt %).

[0012] It is to be noted that “major component” in the context of thisspecification means a component having a largest content on the weightbasis in a target texture (the same will apply to other expressions suchas “mainly” or “mainly comprises”).

[0013] By containing at least one of Ti, Zr and Hf, together with C, andalso with either S or Se, the Ni-base heat-resistant alloy will havegenerated in the matrix thereof a compound (machinability improvingcompound phase) based on such composition. The present inventors foundthat the Ni-base heat-resistant alloy was significantly improved in themachinability by having generated in the matrix thereof suchmachinability improving compound phase, which led us to propose thepresent invention.

[0014] A reason why the machinability of the Ni-base heat-resistantalloy can be improved by the formation of such machinability improvingcompound phase is supposed as follows. That is, when the alloy issubjected to processing such as cutting or grinding in order to remove aportion thereof, the machinability improving compound phase finelydispersed in the matrix can act just like a perforation to therebyfacilitate formation of the sectional plane, which is supposed as beingresponsible for the improved machinability. Any way, the machinabilityimproving compound phase can be responsible for a machinabilityequivalent to or superior to that attainable by the foregoingmachinability improving elements which have previously been used, whilesuccessfully avoiding degradation of other characteristics inherent tothe heat-resistant alloy and retaining a good hot workability.

[0015] In the conventional Ni-base heat-resistant alloy, it has beenconsidered as necessary to intentionally control the content of sulfur(S) in order to keep a good hot workability, and in some cases even aneffort has been made to use a high-purity Ni material containing almostno S. On the contrary in the present invention, such S content will bein an allowable range since the S will be incorporated into suchmachinability improving compound phase as one constituent thereof. Sothat S contained in the Ni-base heat-resistant alloy of the presentinvention will not heavily affect the hot workability of the alloy. Thismakes it possible to use a source material containing a relatively largeamount of S, which is expected to result in an improved productivity.

[0016] A reason why the hot workability of the conventional Ni-baseheat-resistant alloy degraded due to the addition of S can be explainedby the formation of (Ni, S) compound, in particular Ni₃S₂ in the alloy.In the present invention, S contained in the alloy is incorporated intothe machinability improving compound phase during the growth thereof,which suppresses the formation of Ni₃S₂ and thus successfully preventthe hot workability from being degraded for its S content.

[0017] Another advantage of the formation of the machinability improvingcompound phase relates to that it hardly affects the strength andcorrosion resistance at high temperature ranges, which are propertiesmost critical for the Ni-base heat-resistant alloy. In this case,properties such as strength and corrosion resistance at high temperatureranges are defined by residual constituents in the matrix other thansuch machinability improving compound phase. So that the heat-resistantalloy will be obtained with desired properties by properly adjusting thecomposition of the matrix other than the machinability improvingcompound phase.

[0018] In the Ni-base heat-resistant alloy of the present invention, themachinability improving compound phase can be generated so as to bedispersed within the matrix. In particular, finer dispersion of suchcompound phase within the matrix will result in better machinability ofthe Ni-base heat-resistant alloy. In order to raise the improving effectof the machinability, it is preferable to control an average size of themachinability improving compound phase observed in the polishedsectional microstructure of the Ni-base heat-resistant alloy (maximumwidth between two parallel tangential lines which are drawn in somedifferent directions so as to circumscribe the outer contour of thecompound grain) within a range from 1 to 5 μm or around.

[0019] An area ratio of the machinability improving compound phaseobserved in a polished surface of the material is preferably 0.1 to 10%.For the purpose of obtaining improving effect of the machinability byforming such machinability improving compound phase, such phase must becontained in an amount of 0.1% or more in terms of an area ratio in thepolished sectional microstructure. Excessively large area ratio willhowever be no more effective due to saturation of such effect, or mayrather adversely affect other characteristics inherent to theheat-resistant alloy (i.e., strength and corrosion resistance at hightemperature ranges). So that the area ratio in the polished sectionalmicrostructure of the Ni-base heat-resistant alloy is preferably set to10% or below.

[0020] The machinability improving compound phase can typically be suchthat mainly comprising a compound expressed by a composition formulaM₄Q₂C₂ (where M represents the metal element containing any one of Ti,Zr and Hf as a major constituent, and Q represents either S or Se) It isto be noted now that in this specification the compound expressed bysuch formula may occasionally be abbreviated as “TICS”. The compound hasa good dispersion property into the matrix, and is particularlyexcellent in raising the machinability.

[0021] As for the component M in the compound, it is more preferablethat Ti is essentially contained, where Zr and/or Hf may optionally becontained. In the case that V, Nb or Ta is contained in the Ni-baseheat-resistant alloy, at least a part of which may compose suchcomponent M. As for the component Q, it is more preferable that S isessentially contained, where Se may be contained so as to substitute fora part of S. Both components M and Q are not precluded from containingany other components than described in the above as subsidiarycomponents in order to obtain the effect of the present invention as faras properties to be possessed by the machinability improving compoundphase (improving machinability and good dispersion property) are notruined. The machinability improving compound phase including V, Nb, Taor so may possibly improve the strength of such compound.

[0022] The M₄Q₂C₂-base compound in the Ni-base heat-resistant alloy canbe identified by X-ray diffractometry and electron probe X-raymicro-analysis (EPMA). For example, presence or absence of theM₄Q₂C₂-base compound can be confirmed based on presence or absence ofthe correspondent peak ascribable to such compound in a measured profileobtained by the X-ray diffractometry. An area where the compound isformed in the alloy can be specified by subjecting the sectionalmicrostructure of the alloy to surface analysis based on EPMA, and thencomparing two-dimensional mapping results of characteristic X-rayintensity ascribable to Ti, Zr, Hf, S, Se or C.

[0023] Next paragraphs will describe causes for specifying ranges ofcontents of the individual components in the Ni-base heat-resistantalloy of the present invention.

[0024] (1) Ni: contained as a major component

[0025] Ni is an essential component for composing the Ni-baseheat-resistant alloy of the present invention, so that it is containedas a major component. Considering the balance with other essentialadditional element components, the upper limit of the content thereof isset to 85 wt %. Ni content does not exceed 85 wt % also in the most ofgenerally available Ni-base heat-resistant alloys, since the contentexceeding 85 wt % may sometimes fail in fully demonstrating theproperties specific to heat-resistant alloys due to relative shortage ofcontents of the residual components. So that the Ni content ispreferably 85 wt % at most, and more preferably 50 to 80 wt %.

[0026] (2) C: 0.01 to 0.3 wt %

[0027] C is an essential element for improving the machinability in thepresent invention. C, in coexistence with (Ti, Zr, Hr) or S describedlater, can form the machinability improving compound phase. The contentof C less than 0.01 wt % will be too short to form the machinabilityimproving compound phase in an amount enough for markedly improving themachinability. On the contrary, the content exceeding 0.3 wt % willincrease a portion of C not contributive to the formation of themachinability improving compound phase, which will result in excessiveproduction of other carbides and carbo-sulfides. Excessive production ofsuch carbides and carbo-sulfides is undesirable since they are causativeof degraded hot workability and ductility. The C content is morepreferably 0.03 to 0.2 wt %.

[0028] Cr: 14 to 35 wt %

[0029] Cr is an important element for ensuring corrosion resistance andoxidation resistance of the Ni-base heat-resistant alloy. Efficientachievement of such effects will be ensured in a content of 14 wt % ormore. The content exceeding 35 wt % will however ruin the phasestability, which results in lowered toughness and degradedanti-oxidative property. The Cr content is more preferably set within arange from 16 to 30 wt %, and still more preferably from 18 to 25 wt %.

[0030] (4) At least one of (Ti, Zr, Hf) in a total amount of 0.1 to 6 wt%

[0031] Ti, Zr or Hf is an essential component element of themachinability improving compound phase which plays a principal role inexhibiting improving effect of the machinability of the free-cuttingNi-base heat-resistant alloy of the present invention. The total contentof at least one of these elements of less than 0.1 wt % will result inan insufficient amount of production of the machinability improvingcompound phase, so that a sufficient improving effect of themachinability cannot be expected. On the contrary, when the total amountis excessive, (Ti, Zr, Hf) may form compounds with other elements tothereby lower the machinability. So that the total content of theseelements is necessarily suppressed to 6 wt % or less. A part of (Ti, Zr,Hf) as the metal component elements composing the machinabilityimproving compound phase may be substituted by Nb or Ta, which elementscan contribute to the formation of γ′ phase to thereby improve thehigh-temperature strength of the Ni-base heat-resistant alloys. Zr andHf are not so much effective in improving the machinability andhigh-temperature strength as compared with Ti, so that of theseelements, it is more preferable to employ Ti as a major component. Inthis case the Ti content is preferably set within a range from 0.1 to 4wt % in order to efficiently obtain such effect. Although Zr and Hf arenot so effective as Ti in improving the machinability andhigh-temperature strength of the alloy, they are advantageous in raisingthe grain boundary strength through segregation within the grainboundary, so that they may be contained to an extent not causative ofattenuating the Ti-related benefit. It is to be noted that composing themetal component of the machinability improving compound phase only withZr and/or Hf can also be effective in improving the machinability andhigh-temperature strength.

[0032] (5) S: 0.015 to 0.5 wt %

[0033] S is an effective element for improving the machinability. Bycontaining S, compounds effective for raising the machinability (e.g.,the foregoing machinability improving compound phase) can be formedwithin the alloy texture. So that the lower limit of the S content isdefined as 0.015 wt %. Excessive addition of Swill however increase aportion of S not involved in the formation of the machinabilityimproving compound phase (referred to as “free S”), which eventuallypromote the formation of (Ni, S) compounds, in particular Ni₃S₂causative of degrading the hot workability. While the amount offormation of the machinability improving compound phase increases withthe S content, excessive formation thereof will degrade propertiesspecific to the heat-resistant alloy. So that the upper limit of the Scontent is defined as 0.5 wt %. To obtain the improving effect of themachinability by such compound to a desirable degree, it is preferableto properly adjust the S content according to the amount of addition ofother constituents of the machinability improving compound phase such asC, Ti, Zr, Hf or so. The free S is preferably as less as possible, andit is desirable to adjust the S content so that almost all portion of Sadded to the Ni-base heat-resistant alloy will compose the machinabilityimproving compound phase.

[0034] The component Q other than S (which herein means Se) may beincluded in the machinability improving compound phase so as to Asubstitute for S composing such phase. In this case, the Se content ispreferably set within a range from 0.0005 to 0.1 wt %. The content lessthan 0.0005 wt % will be meaningless since the effect of the additionwill hardly become clear. On the other hand, the content exceeding 0.1wt % may sometimes degrade the hot workability and other propertiesspecific to the heat-resistant alloy.

[0035] (6) Satisfying relations of:

W_(Ti)+0.53W_(Zr)+0.27W_(Hf)>2W_(C)+0.75W_(S)  formula A

[0036] and

W_(C)>0.37W_(S)  formula B

[0037] where W_(Ti) represents Ti content (wt %), W_(Zr) represents Zrcontent (wt %), W_(Hf) represents Hf content (wt %), W_(C) represents Ccontent (wt %) and W_(S) represents S content (wt %).

[0038] The left side of the formula A represents a parameter reflectingthe total number of (Ti, Zr, Hf) atoms. That is, the foregoingmachinability improving effect by the machinability improving compoundphase is determined based on the total number of atoms (or the molarnumber), not on the total weight of the constituents to be included.Also the right side of the formula A represents a parameter reflectingthe total number of (C, S) atoms. Coefficients for W_(Ti), W_(Zr) andW_(Hf) appear on the left side of the formula A are determined based ona fact that ratio of the number of (Ti, Zr, Hf) atoms per unit weight ofthe alloy is found to be 1:0.53:0.27, and similarly, coefficients forW_(C) and W_(S) appear on the right side of the formula A are determinedbased on a fact that ratio of the number of (C, S) atoms per unit weightof the alloy is found to be 2:0.75. So that it is to be understood thatthe formula A is such that comparing the total numbers of (Ti, Zr, Hf)atoms and (C, S) atoms. Similarly, the formula B can be understood as aformula for comparing the numbers of C and S atoms contained in thealloy.

[0039] Assuming that all parts of (Ti, Zr, Hf, C, S) atoms added to thealloy are to be involved for the formation of TICS expressed by formulaM₄Q₂C₂, satisfying the above formula A expressing (left side)>(rightside) will inevitably mean that a portion of (Ti, Zr, Hf) atoms notcontributing to the formation of TICS can remain in the residual alloypart. Such portions of (Ti, Zr, Hf) will however hardly affect theproperties of the heat-resistant alloy even they remain in the residualalloy part to some extent, or rather, they may compose the γ′ phase tothereby raise the strength. On the contrary in the case of (leftside)<(right side), a portion of at least either of (C, S) atoms willnever contribute to the formation of TICS and remain in the residualalloy part in a free form. Free S remaining in the residual alloy partis undesirable since it may react with Ni to thereby form (Ni, S)compound, in particular Ni₃S₂, causative of degrading the hotworkability. On the other hand, C which is present in the residual alloypart other than the machinability improving compound phase may degradethe machinability or properties specific to the heat-resistant alloy dueto promoted formation of carbides other than such machinabilityimproving compound. Thus the formula A is necessarily be satisfied.

[0040] Further satisfying herein the formula B ensures that the numberof S atoms to be contained is smaller than that of C. This ensures thatS to be contained will almost completely be fixed to the machinabilityimproving compound phase, and will suppress the content of free Sresiding in the matrix other than such machinability improving compoundphase. A portion of C not involved in the formation of the machinabilityimproving compound phase may sometimes result in the formation ofcarbides responsible for raising the creep strength. This is why theformula B is defined at least as (left side)>(right side). However ashas been described in the above, excessive free C may degrade themachinability or other properties of the alloy, so that it is morepreferable to satisfy the following formula:

0.37W_(S)+0.1>W_(C)  formula B′

[0041] in order to suppress the excessive free C.

[0042] In the free-cutting Ni-base heat-resistant alloy of the presentinvention, the Si content is preferably set to 4 wt % or less, and Mn to1 wt % or less.

[0043] (7) Si : 4 wt % or less

[0044] Si is inevitably contained in the alloy as a deoxidizing element.Intentional addition thereof to a certain extent will be also allowablesince the element has an improving effect of the oxidation resistance ofthe Ni-base heat-resistant alloy. To obtain the oxidation resistance toa sufficient degree, the addition in an amount of at least 0.1 wt % isrecommendable. It is also recommendable to suppress the content to 4 wt% or less since excessive content thereof will degrade the hotworkability and ductility.

[0045] Mn: 1 wt % or less

[0046] Mn is inevitably contained in the alloy as a deoxidizing element.Excessive content thereof however is not desirable since it may not onlydegrade the corrosion resistance but also promote the deposition ofNi₃Ti which is a phase responsible for embrittlement. So that thecontent thereof is preferably suppressed to 1 wt % or less.

[0047] The alloy of the present invention may further contain 0.1 to 5wt % of Al in order to improve the high-temperature strength andcorrosion resistance.

[0048] (9) Al: 0.1 to 5 wt %

[0049] In the Ni-base heat-resistant alloy, Al is responsible for solidsolution hardening by forming solid solution in the matrix thereof, orfor precipitation hardening of γ′ phase by forming γ′ phase (Ni₃Al) byreacting with the Ni component. Al which can form solid solution in thealloy is also expectable for its effect of improving the oxidationresistance at high temperature ranges. The high-temperature strength ofthe Ni-base heat-resistant alloy is often largely contributed especiallyby precipitation hardening of such γ′ phase formation. So that the Alcontent within the above range is preferable in view of obtainingdesirable properties specific to the heat-resistant alloy. Al content ofless than 0.1 wt % results in the foregoing effect only to aninsufficient degree. On the other hand, the content exceeding 5 wt %will inhibit the hot working, so that the Al content is more preferablyset within a range from 0.2 to 3 wt %.

[0050] The Ni-base heat-resistant alloy of the present invention cancontain at least any one of 0.1 to 20 wt % of Co, 0.1 to 20 wt % of Moand 0.1 to 20 wt % of W.

[0051] (10) Co: 0.1 to 20 wt %

[0052] Similarly to Ni, Co can stabilize the austenitic phase, andincreases the amount of formation of the γ′ phase, which is aprecipitation hardening phase, to thereby improve the strength of thealloy. Co may sometimes improve the high-temperature strength of thealloy by forming solid solution in the Ni component. To obtain theeffect of addition to a desirable degree, the Co content is preferablyset to 0.1 wt % or above. On the other hand, the addition exceeding 20wt % is no more desirable since the effect of solid solution hardeningwill saturate, and the cost will increase.

[0053] (11) Mo: 0.1 to 20 wt %; W: 0.1 to 20 wt %

[0054] Mo and W are responsible for improving high-temperature strengthof the alloy by forming solid solution in the texture thereof, and forimproving corrosion resistance based on passivation enhancement. Thecontents less than 0.1 wt % will fail in obtaining a sufficient effect,and on the contrary exceeding 20 wt % will undesirably ruin the hotworkability of the alloy.

[0055] It is further preferable in the present invention to suppress theFe content to 20 wt % or less. Fe is often used as the basic componentof the Ni-base heat-resistant alloy as well as Ni and Cr, but this islargely because Fe is relatively easy to handle and inexpensive.Increasing the Fe content while making a great account of cost hashowever degraded the corrosion resistance of the Ni-base heat-resistantalloy due to relative decrease in the Ni and Cr contents. So that forthe applications in which the corrosion resistance is of a greatimportance, the Fe content is preferably suppressed to 20 wt % or less.Further, the Fe content is preferably suppressed to 10 wt % or less andmore preferably 5 wt % or less.

[0056] The Ni-base heat-resistant alloy of the present invention mayalso contain 0.1 to 5 wt % of Cu. Cu is advantageous in improving thecorrosion resistance, in particular that in the reductive acidicenvironment (in particular sulfuric acid environment), and also inreducing the work hardening property to thereby improve the workability.Cu can also be added in order to improve the antibacterial property,which can be enhanced by annealing. The Cu content is necessarily set to0.1 wt % or above to ensure such effects. The excessive addition howeverdegrades the hot workability, so that the content is preferably setwithin a range of 5 wt % or below.

[0057] The Ni-base heat-resistant alloy of the present invention mayalso contain Nb and Ta in a total amount of 0.1 to 7 wt %. Suchcomponents added to the alloy will form solid solution in the γ′ phase(Ni₃Al) formed in the texture of the Ni-base heat-resistant alloy, tothereby raise the strength of such γ′ phase (Ni₃Al), and thus raise thehigh-temperature strength of the entire alloy. Such components can alsobe included in the foregoing machinability improving compound phase tothereby increase the strength thereof. To obtain such effect to adesirable extent, the total content thereof is preferably set to 0.1 wt% or above. On the contrary, the content exceeding 7 wt % willundesirably degrade the toughness. More preferable total amount of Nband Ta resides within a range from 0.5 to 5 wt %.

[0058] The Ni-base heat-resistant alloy of the present invention mayalso contain 0.0005 to 0.01 wt % of B. B is a valuable element forimproving the hot workability. The content less than 0.0005 wt % willresult in only a limited range of effects, and exceeding 0.01 wt % willdegrade the hot workability.

[0059] Specific examples of materials applicable to the Ni-baseheat-resistant alloy of the present invention will be listed below (allin trade names). It is to be defined that the alloy compositions thereofare such that containing machinability improving elements (Ti, Zr, Hf,S, Se, C, etc.) specified in the present invention so as to substitutefor a part of Ni as a major component. So that, the names listed belowmean specific alloys of the present invention derived from the alloyswhose composition are specified by the product standard, although theproduct names were quoted herein for convenience. Individual alloycompositions of the original products are described in “Kinzoku DetaBukku (Metal Data Book), 3rd edition.”, p. 138, published by Maruzen,and will not be detailed in this specification.

[0060] (1) Solution-hardened Ni-base heat-resistant alloy:Hastelloy-C22, Hastelloy-C276, Hastelloy-G30, Hastelloy X, Inconel 600and KSN.

[0061] (2) Precipitation-hardening Ni-base heat-resistant alloy:Astroloy, Cabot 214, D-979, Hastelloy S, Hastelloy XR, Haynes 230,Inconel 587, Inconel 597, Inconel 601, Inconel 617, Inconel 625, Inconel706, Inconel 718, Inconel X750, M-252, Nimonic 75, Nimonic 80A, Nimonic90, Nimonic 105, Nimonic 115, Nimonic 263, Nimonic PE.11, Nimonic PE.16,Nimonic PK.33, Rene 41, Rene 95, SSS 113MA, Udimet 400, Udimet 500,Udimet 520, Udimet 630, Udiment 700, Udimet 710, Udimet 720, Unitemp AF2-1 DA 6 and Waspaloy.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLES

[0062] The following experiments were carried out to investigate theeffects of the present invention.

[0063] The individual alloys of the present invention and comparativealloys respectively having compositions listed in Tables 1 and 2 weremelted in a vacuum induction heater to thereby obtain 50-kg alloyingots. The ingots were then kept at 1,200° C. for homogenization, andwere then processed by hot forging within a temperature range from 1,200to 1,000° C. to thereby obtain round rods of 65 mm in diameter. A partof such rods was further forged to reduce the diameter to as small as 20mm. The rods were then subjected to solution heat treatment at 1,100° C.for 1 hour, and then successively to age hardening at 700° C. for 16hours. The 65-mm-diameter rods were subjected to machinabilityevaluation, and the 20-mm-diameter rods were subjected to evaluation ofhot workability, hardness after aging and creep characteristics. TABLE 1No. C Si Mn S Se Cr Ti Zr Hf M Al Co Mo W Fe Cu Nb + Ta B Example 1 0.030.16 0.12 0.030 — 19.2 2.53 0.12 — 2.65 1.58 — — — — — — — 2 0.08 0.720.14 0.043 — 32.4 0.27 — 0.20 0.27 2.16 — — — — — 2.2 — 3 0.06 0.22 0.080.098 — 22.4 0.56 — — 0.56 0.12 — 15.3 — 2.2 0.3 3.8 — 4 0.11 0.35 0.290.102 — 19.8 2.64 0.09 — 2.73 1.40 15.5 — — — — — 0.003 5 0.19 3.32 0.560.307 — 28.3 0.72 0.00 — 0.72 1.98 — — 15.2 1.8 — — — 6 0.22 0.14 0.160.419 — 23.5 3.16 — — 3.16 1.58 — 3.4 1.5 — — — — 7 0.09 0.50 0.53 0.216— 18.4 2.69 0.15 0.07 2.84 1.69 — 2.6 — 16.2 — — — 8 0.14 0.82 0.510.163 — 21.0 2.35 — — 2.35 0.89 — — — 5.6 0.23 1.7 0.006 9 0.06 0.340.19 0.087 0.01 14.3 2.12 0.20 — 2.32 1.14 5.7 2.3 — — — 0.5 — 10 0.050.49 0.27 0.114 — 25.1 2.81 — — 2.81 1.39 — — 2.7 3.4 3.8 — — 11 0.10.11 0.07 0.061 — 14.6 1.98 — — 1.98 4.35 — 4 — — — — — Compara- 12 0.040.23 0.20 0.001 — 20.6 2.51 0.09 — 2.6 1.55 — — — — — — — tive 13 0.070.69 0.12 0.005 — 31.8 0.26 — 0.22 0.26 2.16 — — — — — 2.1 — Example 140.02 0.22 0.10 0.530 — 22.4 0.55 — — 0.55 0.13 — 15.4 0 1.9 0.5 3.9 — 150.35 0.32 0.31 0.024 — 20.2 2.67 0.12 — 2.79 1.43 15.2 — — — — — 0.00216 0.01 0.19 0.22 0.055 — 25.9 2.3 0.14 — 2.44 1.57 — 1.8 — 5.3 — —0.003 17 0.42 3.26 0.55 0.771 — 31.9 0.69 — — 0.69 1.92 — — 15.4 1.6 — —— 18 0.03 0.23 0.26 0.111 — 20.5 0.07 — — 0.07 2.89 8.7 5.2 — 0.4 — — —19 0.14 0.07 0.09 0.169 — 24.8 5.33 0.5 0.41 6.24 1.65 — — — — — 0.80.003 20 0.05 0.15 0.18 0.073 — 11.7 2.73 — — 2.73 1.6 10.4 — — — — 1.2— 21 0.06 0.11 0.08 0.095 — 38.6 2.67 — — 2.67 1.44 — 1.9 — — — — — 220.13 6.78 0.19 0.123 — 27.2 2.84 0.07 — 2.91 1.34 — — — 0.8 — — — 230.16 0.14 3.77 0.216 — 21 3.22 — 0.13 3.35 1.51 — 3.3 1.2 — — — 0.005 240.21 0.12 0.34 0.398 — 18.8 3.14 — — 3.14 0.8 — — — — — — — 25 0.06 0.140.12 0.047 — 20.3 2.85 — — 2.85 2.62 — — — — — — 0.002

[0064] TABLE 2 No. Ti + 0.53 Zr + 0.27 Hf 2C + 0.75S Formula A 0.37SFormula B Formula B′ Example 1 2.59 0.08 ◯ 0.01 ◯ ◯ 2 0.32 0.19 ◯ 0.02 ◯◯ 3 0.56 0.19 ◯ 0.04 ◯ ◯ 4 2.69 0.30 ◯ 0.04 ◯ ◯ 5 0.72 0.61 ◯ 0.11 ◯ ◯ 63.16 0.75 ◯ 0.16 ◯ ◯ 7 2.79 0.34 ◯ 0.08 ◯ ◯ 8 2.35 0.40 ◯ 0.06 ◯ ◯ 92.23 0.19 ◯ 0.03 ◯ ◯ 10 2.81 0.19 ◯ 0.04 ◯ ◯ 11 1.98 0.25 ◯ 0.02 ◯ ◯Comparative 12 2.56 0.08 ◯ 0.00 ◯ ◯ Low-S version of No. 1 Example 130.32 0.14 ◯ 0.00 ◯ ◯ Low-S version of No. 2 14 0.55 0.44 ◯ 0.20 X ◯High-S version of No. 3 15 2.67 0.72 ◯ 0.01 ◯ X High-C version of No. 416 2.37 0.06 ◯ 0.02 X ◯ Low-C 17 0.69 1.42 X 0.29 ◯ X High-C, S versionof No. 5 18 0.07 0.14 X 0.04 X ◯ M < 0.1% 19 5.87 0.41 ◯ 0.06 ◯ ◯ M > 6%20 2.73 0.15 ◯ 0.03 ◯ ◯ Cr < 14% 21 2.84 0.19 ◯ 0.04 ◯ ◯ Cr > 35% 222.88 0.35 ◯ 0.05 ◯ ◯ Si > 4% 23 3.26 0.48 ◯ 0.08 ◯ ◯ Mn > 1% 24 3.140.71 ◯ 0.15 ◯ ◯ Al < 0.1% 25 2.85 0.16 ◯ 0.02 ◯ ◯ Al > 5%

[0065] While a major inclusion in the alloy of the present invention wasfound to be a compound expressed as (Ti, Zr, Hf)₄S₂C₂ (TICS), somealloys were also found to include (Ti, Zr, Hf)-base sulfide such as (Ti,Zr, Hf)S, or (Ti, Zr, Hf)-base carbide such as (Ti, Zr, Hf)C. There wasalmost no sign of presence of Ni-S compounds, in particular Ni₃S₂, inthe Ni-base heat-resistant alloy of the present invention.

[0066] Such inclusions were identified by the following procedure.

[0067] Each round rod was cut to produce a proper amount of test pieces,and the metal matrix thereof was dissolved by an electrolytic processusing a methanol solution containing tetramethylammonium chloride and10% actylacetone as an electrolyte. The electrolytic solution after thesolubilization was filtered to thereby extract the insoluble compoundcontained in the Ni-base alloy. The extracted compound was dried, andwas then analyzed by X-ray diffractometry for identification based onobserved peaks in the diffraction profile. The composition of thecompound grains in the alloy was separately analyzed by EPMA. Atwo-dimensional mapping based on the EPMA analysis proved formation of acompound having a composition corresponded to that of a compoundidentified by the X-ray diffractometry.

[0068] The individual test pieces were then subjected to each of thefollowing experiments.

[0069] 1. Machinability Test

[0070] Machinability was evaluated based on the amount of wear of thetool when the test piece was cut, and on roughness of the cut surface. Amachining tool employed was made of a cemented carbide, with which wetcutting was performed at a peripheral speed of 30 m/min, feed perrevolution of 0.2 mm, and depth of cut per revolution of 1.5 mm. Theamount of wear of the tool was defined by flank wear on the machiningtool after 30 minutes of cutting. Roughness of the cut surface wasobtained by measuring arithmetical mean (Ra: μm) of the sample surfaceafter the cutting based on JIS-B0601.

[0071] 2. Hot Workability Evaluation

[0072] A test piece of 6 mm in diameter was cut from the 20-mm-diameterrod, and then subjected to tensile test to thereby evaluate the hotworkability. The test was performed using a high-speed tension tester atvarious temperatures from 900 to 1,250° C., and tensile speed of 50mm/sec. Defining now the hot workable range as a temperature range inwhich rupture drawing of not less than 40%, which is a required valuefor allowing forging, is ensured, the samples having such temperaturerange of 200° C. or more were assessed as “excellent in hot workability(◯)”, and those having such temperature range of less than 200° C. wereassessed as “poor in hot workability (X)”.

[0073] 3. Hardness Test

[0074] C-scale Rockwell hardness of the Ni-base heat-resistant alloy wasmeasured at room temperature according to the Rockwell hardness testingprocedures specified in JIS-Z2245.

[0075] 4. High-Temperature Strength Evaluation

[0076] The high-temperature strength was evaluated by carrying out creeprupture test based on the method specified by JIS-Z2272. Morespecifically, a test piece of 6 mm in diameter was cut from the20-mm-diameter rod, and then subjected to creep test at 700° C. under a400-MPa load, and the duration of time before the test piece ruptureswas measured.

[0077] Experimental results of these tests were shown together in Table3. TABLE 3 Hardness Cutting Test Hot workability Temperature afterRoughness of cut range ensuring 40% or aging Creep rupture No. Flankwear (μm) surface (μm) more drawing of 200° C. or above (HRC) time (hr)1 183 3.8 ◯ 37.8 287 2 132 3.4 ◯ 32.3 141 3 178 3.4 ◯ 30.1 93 4 167 3.2◯ 38.4 304 5 154 3.0 ◯ 33.0 150 6 124 3.5 ◯ 41.6 342 7 149 3.1 ◯ 38.2295 8 131 3.3 ◯ 32.5 149 9 170 3.4 ◯ 35.3 216 10 165 3.2 ◯ 39.1 324 11196 3.4 ◯ 44.9 418 12 312 8.2 ◯ 37.4 278 13 299 7.8 ◯ 32.1 134 14 1863.4 X 30.3 89 15 238 8.4 X 37.9 298 16 197 3.7 X 33.2 143 17 225 4.3 X26.8 75 18 257 4.6 X 30.7 97 19 155 3.5 X 41.6 241 20 194 3.8 ◯ 38.9 10621 231 5.4 X 50.3 332 22 189 3.9 X 40.1 223 23 143 3.2 ◯ 39.4 188 24 1363.2 ◯ 20.8 44 25 192 4 X 44.5 256

[0078] It was made clear from Table 3 that the Ni-base heat-resistantalloy of the present invention in Examples 1 to 11 showed excellenthardness after aging at room temperature and creep characteristics athigh temperature ranges, which proved satisfactory characteristicsspecific to the heat-resistant alloy, and excellent machinability aswell. On the contrary, Comparative Examples 12 and 13 showed only poormachinability, which was ascribable to insufficient formation of TICS,which is the machinability improving compound phase, due to an extremelylow S content. Comparative Example 14 showed an excellent machinabilityby the formation of TICS, but was found to be poor in the hotworkability due to an excessive S content. Comparative Example 15 showedan excellent creep characteristic at a high temperature range, but wasfound to be poor in the machinability and hot workability due to anexcessive C content. Comparative Example 18 showed only a poormachinability, which was ascribable to insufficient formation of TICSdue to an extremely low total contents (M) of (Ti, Zr, Hf), and wasfound also poor in the hot workability since S cannot be fixed by TICS.Comparative Example 19 showed only a poor hot workability due toexcessive M.

[0079] It was thus concluded that the Ni-base heat-resistant alloy ofthe present invention can successfully improve the machinability withoutruining the hot workability, while retaining the other characteristicsspecific to the heat-resistant alloy as comparable to those of theconventional heat-resistant alloys.

What is claimed is:
 1. A free-cutting Ni-base heat-resistant alloycontaining Ni as a major component; containing C in an amount of 0.01 to0.3 wt % and Cr in an amount of 14 to 35 wt %; containing at least oneelement selected from Ti, Zr and Hf in a total amount of 0.1 to 6 wt %,and S in an amount of 0.015 to 0.5 wt %; having dispersed in the matrixthereof a machinability improving compound phase, said phase containingany one of Ti, Zr and Hf as a major constituent of the metal elements,essentially containing C and either S or Se as a binding component forsuch metal elements; and satisfying the relations of:W_(Ti)+0.53W_(Zr)+0.27W_(Hf)>2W_(C)+0.75W_(S); andW_(C)>0.37W_(S) whereW_(Ti) represents Ti content (wt %), W_(Zr) represents Zr content (wt%), W_(Hf) represents Hf content (wt %), W_(C) represents C content (wt%) and W_(S) represents S content (wt %).
 2. The free-cutting Ni-baseheat-resistant alloy according to claim 1, wherein said machinabilityimproving compound phase mainly comprises a component phase expressed bya composition formula M₄Q₂C₂ (where M represents the metal elementcomponent containing any one of Ti, Zr and Hf as a major constituent,and Q represents either S or Se).
 3. The free-cutting Ni-baseheat-resistant alloy according to claim 1 further satisfying a relationof 0.37W_(S)+0.1>W_(C).
 4. The free-cutting Ni-base heat-resistant alloyaccording to claim 1 further containing Si in an amount of 4 wt % orless and Mn in an amount of 1 wt % or less.
 5. The free-cutting Ni-baseheat-resistant alloy according to claim 1 further containing Al in anamount of 0.1 to 5 wt %.
 6. The free-cutting Ni-base heat-resistantalloy according to claim 1 further containing at least any one of 0.1 to20 wt % of Co, 0.1 to 20 wt % of Mo and 0.1 to 20 wt % of W.
 7. Thefree-cutting Ni-base heat-resistant alloy according to claim 1 furthercontaining Fe in an amount of 20 wt % or less.
 8. The free-cuttingNi-base heat-resistant alloy according to claim 1 further containing Cuin an amount of 0.1 to 5 wt %.
 9. The free-cutting Ni-baseheat-resistant alloy according to claim 1 further containing Nb and Tain a total amount of 0.1 to 7 wt %.
 10. The free-cutting Ni-baseheat-resistant alloy according to claim 1 further containing B in anamount of 0.0005 to 0.01 wt %.